Hydrodynamics from free-streaming to thermalization and back again

TL;DR

This paper investigates the evolution of hydrodynamic quantities in expanding fluids with increasing mean free path, revealing the role of hydrodynamic attractors during the transition from thermalization to decoupling.

Contribution

It introduces a simple 1D model to study how the Knudsen and Reynolds numbers evolve with time in systems with rapidly increasing mean free paths, highlighting the significance of hydrodynamic attractors.

Findings

Hydrodynamic attractors govern the evolution of key fluid dynamic quantities.

The transition from thermalizing to decoupling is characterized by distinct changes in attractor trajectories.

The model demonstrates the importance of non-traditional mean free path growth in fluid dynamics.

Abstract

We study the evolution of the Knudsen and Reynolds numbers in (0+1)-dimensionally expanding fluids with Bjorken symmetry for systems whose microscopic mean free path rises more quickly with time than usually assumed. This allows us to explore within a simple 1-dimensional model the transition from initially thermalizing to ultimately decoupling dynamics. In all cases studied the dynamics is found to be controlled by hydrodynamic attractors for both the Knudsen and Reynolds numbers whose trajectories undergo characteristic changes as the dynamics changes from thermalizing to decoupling.